For those who work with the public: What was the worst situation that occurred between you and a customer?

Many years ago, I was supervising at a supermarket checkout. An older lady (late 50's / early 60's) started berating one of the young checkout girls, she was newish about 16yo.

I over heard, it was hard not to after a little while. I walked over, I was 19 at the time. I'm not a big guy 5'9" and weighed around 60kg (checks math...132lb). The old woman sees me and begins to go into a rant, to which I simply said, get the fuck out of my store. It was latish (8:30pm), there were no managers left in store.

She went into (what is now called) full karen mode, ranting about calling my manager. I got a bit pf paper and wrote my managers and the store managers numbers down with their names and said, "go ahead, call them". She quickly calmed down and went to go stand back in line....I said, "no I told you to get out".